Method for manufacturing a bipolar transistor having a polysilicon emitter

ABSTRACT

In the inventive method for manufacturing a bipolar transistor having a polysilicon emitter, a collector region of a first conductivity type and, adjoining thereto, a basis region of a second conductivity type will be generated at first. At least one layer of an insulating material will now be applied, wherein the at least one layer is patterned such that at least one section of the basis region is exposed. Next, a layer of a polycrystalline semiconductor material of the first conductivity type, which is heavily doped with doping atoms, will be generated such that the exposed section is essentially covered. Now, a second layer of a highly conductive material on the layer of the polycrystalline semiconductor material will be generated in order to form an emitter double layer with the same. Thereupon, at least part of the doping atoms of the first conductivity type of the heavily doped polycrystalline semiconductor layer is caused to get into the basis region to generate an emitter region of the first conductivity type.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of copending InternationalApplication No. PCT/EP02/08234, filed Jul. 10, 2002, which designatedthe United States and was not published in English.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the manufacture of semiconductordevices and, in particular, to the manufacture of bipolar transistorshaving a polysilicon emitter, which comprise reduced emitter resistance.

2. Description of Prior Art

In bipolar transistors, which are designed to take high powers andspeeds, use is already made of polysilicon emitters. In this connection,with respect to the theoretical and experimental aspects of the use ofbipolar transistors having a polysilicon emitter, reference is made tothe article by C. R. Selvakumar, “Theoretical and Experimental Aspectsof Polysilicon Emitter Bipolar Transistors”, published in IEEE Press,1988, pages 3 to 16.

Therefore, one embodiment of a bipolar transistor having a polysiliconemitter provides a heavily doped polysilicon layer located above thebasis, which serves both as a diffusion source for the generation of aflat (emitter/basis) semiconductor transition and as a means forcontacting the flat emitter region. After performing the conventionalprocessing steps for manufacturing the basis region and the emitterwindow openings, either non-doped or doped polysilicon will be applied,into which, if the polysilicon is non-doped, an exact quantity ofarsenic atoms will be implanted. Thereupon, by way of heat treatment(tempering), damages will be annealed, and the emitter/basissemiconductor transition is formed.

As can be seen from the above-cited article on page 4, one of thecritical processing steps in the manufacture of bipolar transistorshaving polysilicon emitters consists of the treatment of the waverexactly before applying the polysilicon. Therefore, the many differenttreatment methods, as are known in the state of the art, may be roughlysub-divided into two categories. The first treatment refers tointentional or unintentional growth of a thin oxide layer (0.2 to 2 nm).The second treatment refers to the epitaxial growth of a thin thermalnitride layer (approximately 1.0 to 1.5 nm). The “interface” treatmentis important, since the same has strong effects on the electricalcharacteristics of bipolar transistors having a polysilicon emitter.

As has been mentioned above in brief, it is tried to achieve bipolartransistors having high cut-off frequencies and high current gains byforming the emitter(s) of a bipolar transistor by depositing a heavilydoped polysilicon layer. The doping agent in the polysilicon layer willthen diffuse, by way of tempering, from the polysilicon layer into thesingle crystal silicon substrate below, where it forms theelectrically-active emitter area of the bipolar transistor. Here, thepolysilicon used serves as a doping agent source, as a feed and also asa landing surface for the contact terminal holes yet to be formed. Asfor the operational properties of the transistor, the use of polysiliconhas the following decisive advantage that the interface between thepolysilicon layer and the single crystal silicon substrate serves as adiffusion barrier for minority carriers which are injected from thebasis, thus clearly increasing current gain and cut-off frequency of thetransistor.

However, one disadvantage of polysilicon is the specific resistancewhich is by orders of magnitude higher as compared to metals. Therelatively high emitter resistance resulting therefrom especiallyaffects the high-frequency properties of the bipolar transistors. Owingto these problems, it has been tried to use as thin a polysilicon layeras possible. On the other hand, a certain minimum thickness of mostlyfar above 100 nm is required, since an etching of contacting holes forthe contact pads has to stop on this polysilicon layer in order toensure the process safety during the manufacture of bipolar transistors.The problem concerning the emitter resistance still increases withmodern bipolar transistors having very narrow emitter windows, since thepolysilicon used may completely fill the emitter window in this case,and, thus, the height of the polysilicon layer over the active emitterfurther increase

It should be appreciated that, instead of polysilicon, an amorphoussilicon may also be used which, in turn, may rest to crystallize insubsequent tempering processes.

In order to solve the problems shown above concerning the manufacture ofbipolar transistors having a polysilicon emitter, concepts haven beentaken up, which provide a thermal silicidation of the emitter afterdepositing a metal layer. Silicides are metal/silicon compounds, whichare used in silicon technology as temperature-stable, low-resistancetraces and contacts. The silicide layers typically provide a thicknessof 0.1 to 0.2 μm. However, the silicide layer formed in this manner is,as a rule, relatively irregular, thus, in practice, making it impossibleto fill the emitter window with this layer.

As a further measure, the layer thickness of the polysilicon was kept aslow as possible and the doping of same was kept as high as possible. Ifpossible, filling the emitter window with polysilicon was avoided,which, however, in earlier technologies, was much easier owing to largeremitter dimensions. If, after depositing the polysilicon on the emitter,a very narrow gap is left, increased efforts were necessary, dependingon the technology chosen, when etching the contacting holes, since thisgap may be filled with an undesired material, e.g. with a nitridebarrier, when depositing further layers.

In many cases, the negative influence of the emitter resistor on thetransistor properties was simply accepted and/or it was tried tocompensate for this negative influence in terms of circuit technology.

SUMMARY OF THE INVENTION

It is the object of the present invention to provide an improved methodfor manufacturing a bipolar transistor having a polysilicon emitterwhose emitter resistance is clearly reduced in order to improve theelectrical properties of the bipolar transistor.

In accordance with a first aspect, the invention provides method formanufacturing a bipolar transistor having a polysilicon emitter, havingthe following steps: generating a collector region of a firstconductivity type and a basis region of a second conductivity typeadjoining thereto in a semiconductor substrate; applying apolycrystalline layer of a polycrystalline semiconductor material of thesecond conductivity type heavily doped with doping atoms on thesubstrate, so that a portion of the basis region is exposed; applying atleast one insulating layer of an insulating material on thepolycrystalline layer; patterning the at least one insulating layer suchthat at least one section of the basis region is exposed; generating afurther polycrystalline layer of a polycrystalline semiconductormaterial of the first conductivity type heavily doped with a doping atomsuch that the exposed section is essentially covered, wherein thepolycrystalline layer and the further polycrystalline layer are isolatedby the insulating layer; generating a highly conductive layer of ahighly conductive material on the further polycrystalline layer togenerate an emitter double layer with the same; effecting that at leastpart of the doping atoms of the second conductivity type of thepolycrystalline layer get into the semiconductor substrate toelectrically connect the base region to the polycrystalline layer;effecting that at least part of the doping atoms of the firstconductivity type of the heavily doped further polycrystalline layer getinto the basis region to generate an emitter region of the firstconductivity type; structuring the emitter double layer for generatingan emitter terminal area; contacting the emitter terminal area with ancontact terminal.

The present invention is based on the recognition that, by forming anemitter double layer during the manufacture of a bipolar transistorhaving a polysilicon emitter, the specific resistance of the emitterterminal will be reduced and, thus, the electrical characteristics ofthe device will significantly improve with the present invention, theemitter of the bipolar transistor will be deposited in two stages. Here,the first layer consists of a common, heavily doped polysiliconmaterial. This polysilicon layer now still only serves as source for thedoping material and for generating a polysilicon single crystallineinterface between a polysilicon layer and the single crystalsemiconductor material of the substrate. As a result, the polysiliconlayer used may be selected significantly thinner as has been the case sofar. The second layer applied is a layer of a highly conductivematerial, by way of which the lead resistance to the emitter of thebipolar transistor is kept at a low level. This highly conductive layerfurther serves as a stop layer for the etching of the contacting holesto be performed for the various contact pads. This layer may completelyfill the emitter window without having any considerably negative effecton the emitter resistance, i.e. without the emitter resistance beingincreased.

This second highly conductive layer has to sustain the high temperaturesof emitter tempering (of the temperature treatment) of typically about1000° C. or higher and, for reasons based on the manufacturingtechnology, should further provide properties similar to the siliconmaterial used in the various manufacturing processes, such as e.g. indry etching processes.

By way of the inventive method for manufacturing a bipolar transistorhaving a polysilicon emitter, in which a two-layer emitter deposition isprovided in order to form an emitter double layer, which ensures anextremely low emitter resistance, extremely favourable electricallycharacteristics of the transistor may thus be achieved. As a result, thereduction in the emitter resistance achieved by the deposited emitterdouble layer has a positive impact on the cut-off frequency and, ingeneral, also on the voltage and power gain in a circuit.

As already noted, the first, lower layer consists of a polysiliconmaterial, which is effective as a doping agent source for the activetransistor region, wherein the second, upper layer consists of a highlyconductive material, which serves as an etch stopping means for etchingthe contacting holes of the contact pads as well as for the verticalcurrent transport between the contact pads and the silicon emitter.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other object and features of the present invention will becomeclear from the following description taken in conjunction with theaccompanying drawing, in which:

FIG. 1 shows an interim state of the manufacturing process of a bipolartransistor having a polysilicon emitter having a narrow emitter windowafter depositing the emitter polysilicon material and the silicidelayer; and

FIG. 2 shows the state of the manufacturing process of a bipolartransistor having a polysilicon emitter having a narrow emitter windowafter patterning the emitter double layer, after tempering andcontacting.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

With reference to FIGS. 1 and 2, a preferred embodiment of the presentinvention for manufacturing a bipolar transistor having a polysiliconemitter will now be explained in detail.

As shown in FIG. 1, preferably, a single crystal silicon body is used,which serves as a substrate 10 for the bipolar transistor. In thesubstrate 10, a first region 12 of a first conductivity type is formed,with this region 12 being referred to as a collector region below. Inthe substrate 10, a further region 14 of a second conductivity type isfurther formed, which is herein referred to as a basis region 14 below.

In connection with the present invention, the first conductivity typedesignates a so-called n-type doping, while the second conductivity typedesignates a so-called p-type doping. A doping in a semiconductormaterial is referred to as a n-type, if the majority charge carrierstherein are electrons, wherein a doping in a semiconductor material isreferred to as a p-type, if the majority charge carriers therein areholes. In the present invention the conductivity types of the dopingsmay each be selected vice versa.

The basis region 14 adjoins the collector region 12, wherein at leastone section of the basis region 14 is formed between the surface 15 ofthe substrate 10 and the collector region 12. On the surface of thesubstrate 10, a polycrystalline layer 17, e.g. of polysilicon, will beapplied in an appropriate manner, which provides the second conductivitytype (p-type), wherein the basis region 14 in the substrate 10 remainsessentially exposed. In the following, this layer 17 serves as a p-dopedbasis terminal area for the basis region 14.

On the surface of the substrate 10 or therein, one or more layers 16 ofa material, e.g. a dielectric (insulating) material, are formed, whereinthe dielectric layers are patterned such that at least one section ofthe basis region 14 is exposed.

Next, a layer 18 of a polycrystalline semi-material, preferablysilicone, will be applied such that this polysilicon layer 18essentially covers the exposed section of the basis region 14.

Since non-doped polysilicon layers provide a very high resistance(approximately 10⁴ Ωcm), in the present case, the polysilicon layer 18,since the same has an electrically conductive function in thetransistor, will be provided with doping agents, e.g. boron, phosphor orarsenic, in order to achieve the respective doping type, the desireddoping strength and, thus, the desired electrically conductivity of thepolysilicon layer. In order to save an additional doping step, thedoping of the polysilicon layer 18 is generally achieved during thepolysilicon deposition by adding suitable materials. In the presentcase, the polysilicon layer 18 comprises the first conductivity type(n-type).

With the present invention, an already heavily doped polysiliconmaterial is preferably applied, since a further, second layer 20 of ahighly conductive material will be applied directly onto the existingpolysilicon layer 18, in order to form, together with the polysiliconlayer 18, a so-called emitter double layer. The second layer 20consisting of a highly conductive material is normally a silicide layer.Silicides are metal/silicone compounds, which are used astemperature-stable low-resistance materials in silicone technology.These suicide layers typically provide a thickness of 0.1 to 0.2 μm,wherein a thickness of 0.1 to 0.2 μm refers to the thickness depositedon regular faces. Therefore, in the emitter window, the thickness orheight of the silicide layer lies clearly above 0.2 μm, e.g. at 0.5 μm.Most frequently, silicides such as MoSi₂ or WSi₂ are used.

For the explanation of the further steps of the inventive method formanufacturing a bipolar transistor having a polysilicon emitter,reference is now made to FIG. 2. The present semiconductor structurewill now be subjected to a temperature treatment (tempering), such thatat least some of the doping agents from the heavily doped polysiliconlayer 18 diffuse into the single crystal body, i.e. into the substrate10. As a result, the active emitter area 22 forms in the substrate, i.e.especially adjacent to the basis area 14. Thus, at least some part ofthe doping atoms of the first conductivity type of the heavily dopedpolysilicon layer 18 gets into the substrate, to generate, adjacent tothe basis region 14 in the substrate 10, an active emitter region 22 ofthe first conductivity type. The active emitter region 22 extends fromthe interface 15, between the polysilicon layer 18 and the substrate 10,into the semiconductor material of substrate 10.

Furthermore, during temperature treatment, some part of the doping atomsof the second conductivity type of the polysilicon layer 17, which isdoped with this second conductivity type and which has been provided forthe basis terminals 15, gets into the substrate 10, with a large-surfaceconnection to the basis region 14 in the substrate 10 resultingtherefrom.

The term temperature treatment, or tempering, in silicon technologyrefers to the treatment of silicon at increased temperatures in an inertatmosphere, e.g. nitrogen, argon, hydrogen, and forming gas. As aresult, no new layers are grown and no material will be removed, but thelayers already existing and the silicon substrate itself will besubjected to decisive changes. In the present case, the doping agents ofthe first or second conductivity type of the various, differently dopedpolysilicon layers 17, 18 get into the adjoining semiconductor materialof the semiconductor substrate 10.

Next, the emitter double layer consisting of the polysilicon layer 18and the highly conductive second layer 20 will be patterned to generatean emitter terminal area of the bipolar transistor. Patterning isusually effected by dry etching of the respective layers. Processing isfacilitated, if the upper, highly conductive silicide layer 20essentially comprises the same or comparative processing properties,e.g. etching properties, such as the polysilicon layer 18.

The exposed sections on the remaining present semiconductor structureare now usually filled with an encapsulating insulating material 28.Further, the so-called contacting holes will be etched in order toprovide the contact terminal 24 for the emitter terminal area and thecontact terminals 26 for the basis terminal area 17. In the emitterterminal area, the silicide layer 20 serves as an etch stopping meansfor etching the contacting holes.

By way of the above-described inventive manufacture of a bipolartransistor having a polysilicon emitter, it is possible to clearlyreduce the specific resistance of the emitter terminal area, whereby theelectrical characteristics of a bipolar transistor may be considerablyimproved. As a result, the reduction in the emitter resistance achievedby way of the emitter double layer deposited has a positive effect onthe cut-off frequency and, in general, also on the voltage and powergain in a circuit.

The inventive, advantageous concept for manufacturing a bipolartransistor having a polysilicon emitter essentially comprises the stepof performing the deposition of the emitter terminal area in two stages.As a result, the first layer 18 consists of the usual heavily dopedpolysilicon material. In the present invention, it merely serves as asource for the doping agent and for generating the polysilicon singlecrystal interface and may, therefore, be selected thinner than before.The second layer 20 is a layer of a highly conductive material whichkeeps the lead resistance at a low level and which serves as a stoppinglayer for etching the contacting holes. It can fill the emitter windowwithout significantly increasing the emitter resistance. In thepreferred processing, this second layer 20 has to sustain the hightemperatures of emitter tempering, which are typically about 1000° C. ormore, and, for simplifying the processing in dry etching processes, mayprovide comparative properties such as the silicone material.

In principal, a type of processing is also conceivable in which emittertempering occurs before depositing the second layer. When depositing thesecond layer, both pure metals and metal silicon compounds (silicides)are ideal, wherein, in particular, all silicides of high-melting-pointmetals, such as e.g. tungsten-disilicide and molybdenum-disilicide, areused. For other materials, such as e.g. tungsten, an additionaldeposition of a diffusion barrier may be necessary.

In the inventive method for manufacturing a bipolar transistor having apolysilicon emitter, a two-stage emitter deposition will be performed inaccordance with the invention, with the lower layer of polysilicon beingeffective as a doping agent source for the active transistor region andthe upper highly conductive layer being effective as an etch stoppingmeans for etching the contacting holes and also for the vertical currenttransport between the contacting hole and the polysilicon emitter.

Further, it should be appreciated that the present invention is alsoapplicable to deviating transistor architectures, in particular, thosehaving an epitaxially grown basis area. Therefore, there are transistorarchitectures, in which the basis area and sometimes also part of thecollector area are epitatically grown onto the substrate. In thesearchitectures, which, in the future, are very likely to be used morefrequently, the inventive emitter double layer may also be used in anadvantageous manner.

While this invention has been described in terms of several preferredembodiments, there are alterations, permutations, and equivalents whichfall within the scope of this invention. It should also be noted thatthere are many alternative ways of implementing the methods andcompositions of the present invention. It is therefore intended that thefollowing appended claims be interpreted as including all suchalterations, permutations, and equivalents as fall within the truespirit and scope of the present invention.

1. Method for manufacturing a bipolar transistor having a polysiliconemitter, comprising: generating a collector region of a firstconductivity type and a basis region of a second conductivity typeadjoining thereto in a semiconductor substrate; applying apolycrystalline layer of a polycrystalline semiconductor material of thesecond conductivity type doped with doping atoms on the substrate, sothat a portion of the basis region is exposed; applying at least oneinsulating layer of an insulating material on the polycrystalline layer;patterning the at least one insulating layer such that at least onesection of the basis region is exposed; generating a furtherpolycrystalline layer of a polycrystalline semiconductor material of thefirst conductivity type heavily doped with doping atoms such that theexposed section is essentially covered, wherein the polycrystallinelayer and the further polycrystalline layer are isolated by theinsulating layer; generating a highly conductive layer of a highlyconductive material on the further polycrystalline layer to form anemitter double layer with the same; effecting, via a temperaturetreatment, that at least part of the doping atoms of the secondconductivity type of the polycrystalline layer get into thesemiconductor substrate to electrically connect the base region to thepolycrystalline layer; effecting that at least part of the doping atomsof the first conductivity type of the heavily doped furtherpolycrystalline layer get into the basis region to generate an emitterregion of the first conductivity type; structuring the emitter doublelayer for generating an emitter terminal area; contacting the emitterterminal area with an contact terminal, wherein the layer and thecontact terminal vary, comprising the following substeps: applying aninsulating material on the emitter terminal area; and etching a contactvia into the insulating material, wherein the highly conductive layer inthe emitter terminal area is effective as stop layer for the viaetching.
 2. Method in accordance with claim 1, wherein the step ofeffecting that at least part of the doping atoms of the firstconductivity type of the heavily doped further polycristalline layergets into the basis region, will be performed by means of tempering. 3.Method in accordance with claim 1, wherein the highly conductive layerconsists of a material having comparative processing properties as thesemiconductor material.
 4. Method in accordance with claim 1, whereinthe step of effecting that at least part of the doping atoms of thefirst conductivity type of the heavily doped further polycrystallinelayer get into the basis region will be performed before or after thestep of generating the highly conductive layer.